diff --git a/packages/backend/tests/helpers/uuid-validation.test.ts b/packages/backend/tests/helpers/uuid-validation.test.ts
new file mode 100644
index 0000000..5f7c6e7
--- /dev/null
+++ b/packages/backend/tests/helpers/uuid-validation.test.ts
@@ -0,0 +1,24 @@
+/**
+ * UUID Validation Helper - Unit Tests
+ * Verifies the UUID validation helper utility exports correctly
+ */
+
+import { describe, expect, it } from 'vitest';
+import { testInvalidUuidRejection } from './uuid-validation.js';
+
+describe('UUID Validation Helper', () => {
+  it('should export testInvalidUuidRejection function', () => {
+    expect(testInvalidUuidRejection).toBeDefined();
+    expect(typeof testInvalidUuidRejection).toBe('function');
+  });
+
+  it('should have correct function signature (2 required params, 1 optional)', () => {
+    // Function.length returns number of required parameters
+    expect(testInvalidUuidRejection.length).toBe(2);
+  });
+
+  it('should be importable from helpers directory', () => {
+    // This test passing means the import worked correctly
+    expect(testInvalidUuidRejection).toBeTruthy();
+  });
+});
diff --git a/packages/backend/tests/helpers/uuid-validation.ts b/packages/backend/tests/helpers/uuid-validation.ts
new file mode 100644
index 0000000..e991ea5
--- /dev/null
+++ b/packages/backend/tests/helpers/uuid-validation.ts
@@ -0,0 +1,63 @@
+import type { FastifyInstance } from 'fastify';
+import { describe, expect, it } from 'vitest';
+
+/**
+ * Shared test helper for UUID validation in API endpoints
+ *
+ * Creates a test suite that verifies:
+ * - Invalid UUID format returns 400 with VALIDATION_ERROR
+ * - Missing UUID returns 404 (route not found)
+ * - Valid UUID v4 is accepted (returns 200 or 404 if entity doesn't exist)
+ *
+ * @param app - Fastify application instance
+ * @param endpoint - API endpoint URL with :id placeholder (e.g., '/api/v1/projects/:id')
+ * @param method - HTTP method to test (default: 'GET')
+ *
+ * @example
+ * ```typescript
+ * import { testInvalidUuidRejection } from '../../helpers/uuid-validation';
+ *
+ * describe('GET /api/v1/projects/:projectId', () => {
+ *   testInvalidUuidRejection(app, '/api/v1/projects/:id', 'GET');
+ *
+ *   // Endpoint-specific tests continue...
+ * });
+ * ```
+ */
+export function testInvalidUuidRejection(
+  app: FastifyInstance,
+  endpoint: string,
+  method: 'GET' | 'POST' | 'PUT' | 'DELETE' = 'GET',
+) {
+  describe('UUID validation', () => {
+    it('should return 400 for invalid UUID format', async () => {
+      const response = await app.inject({
+        method,
+        url: endpoint.replace(':id', 'invalid-uuid'),
+      });
+
+      expect(response.statusCode).toBe(400);
+      expect(response.json().error.code).toBe('VALIDATION_ERROR');
+    });
+
+    it('should return 400 for missing UUID', async () => {
+      const response = await app.inject({
+        method,
+        url: endpoint.replace(':id', ''),
+      });
+
+      expect(response.statusCode).toBe(404); // Route not found
+    });
+
+    it('should accept valid UUID v4', async () => {
+      const validUuid = '550e8400-e29b-41d4-a716-446655440000';
+      const response = await app.inject({
+        method,
+        url: endpoint.replace(':id', validUuid),
+      });
+
+      // Should not fail on UUID validation (may 404 if entity doesn't exist)
+      expect([200, 404]).toContain(response.statusCode);
+    });
+  });
+}